1. Field of the Invention
The present invention relates to a magnetic linear position sensor for detecting linear displacement in a non-contact manner using a magnetoelectric transducer such as a Hall element which transforms a change in a magnetic flux into a voltage.
2. Description of the Related Art
Referring to FIG. 6, a conventional magnetic linear position sensor will be described. A sensor gap 24 is made between two stator yokes 20 and 22 which are made of soft magnetic material. A Hall element 26 is placed in the sensor gap 24, and the Hall element 26 outputs a voltage in accordance with change in a magnetic flux. The stator yokes 20 and 22 and the Hall element 26 form a stator. A permanent magnet 30 is provided through a principal air gap 28 that is perpendicular to the sensor gap 24, and the permanent magnet 30 is magnetized to the N-pole and the S-pole in the thickness direction. The permanent magnet 30 and a yoke 34 which is made of soft magnetic material are fixed. Two leg portions 32 of the yoke 34 oppose the stator with nearly the same air gap as the principal air gap 28. A slider 36 is attached to the yoke 34, and the permanent magnet 30 and the yoke 34 form a mover which can move linearly along the stator.
In this magnetic linear position sensor, in the case where the mover is positioned at the center of the stator as shown in FIG. 6, the permanent magnet 30, the yoke 34, the leg portions 32, and the stator yokes 20 and 22 form magnetic circuits A and B, and the magnetic flux passing through the Hall element 26 is zero. When the mover moves to the right, the balance between the magnetic circuits A and B is lost and the magnetic flux of the magnetic circuit A passes through the Hall element 26. The magnetic flux passing through the Hall element 26 increases gradually as the mover moves to the right side from the state shown in FIG. 6 and then becomes the maximum at the state shown in FIG. 7. Further, when the mover moves to the left, the magnetic flux of the magnetic circuit B passes through the Hall element 26. The magnetic flux passing through the Hall element 26 increases gradually as the mover moves to the left side from the state shown in FIG. 6 and then becomes the maximum at the state shown in FIG. 8. In this case, since the magnetic flux of the magnetic circuit B is opposite in direction to the magnetic flux of the magnetic circuit A, if the move to the right by the slider 36 as shown in FIG. 7 is put into a plus, the move to the left by the slider 36 as shown in FIG. 8 is detected as a minus. Thus, the Hall element 26 detects the magnetic flux, and thereby linear displacement can be detected in a non-contact manner.
However, since lines of the magnetic flux pass through two stator yokes 20 and 22 and the lines of the magnetic flux change in direction and in polarity from the N-pole to the S-pole as the permanent magnet 30 of the mover moves, due to magnetizing properties of soft magnetic material of the stator yokes 20 and 22, a hysteresis phenomenon occurs. That is, there occurs a difference between the output voltages of the Hall element 26 at the same position in the case where the mover moves to one side and in the case where the mover returns and moves to the other side.
In general, hysteresis of about 0.5% of full scale remains in the case of using soft magnetic iron as the soft magnetic material, and hysteresis of about 0.2% remains in the case of using a high-grade silicon steel plate, and hysteresis of about 0.1% remains even in the case of using higher-grade magnetic material such as permalloy; therefore, there is a problem for an application that high degree of detection accuracy is required for.
It is a possible solution of this problem to reduce the difference of the output voltages by a hysteresis characteristic of the soft magnetic material. However, it is very difficult to decrease or eliminate the hysteresis in this kind of magnetic circuit structure. For example, using high-grade magnetic material and adding anneal processing in order to reduce the hysteresis increase the cost of the whole magnetic linear position sensor. Furthermore, in the case of an application of emphasizing reproducibility of a detection value as a demand characteristic to the magnetic linear position sensor, even hysteresis of the order of 0.1% cannot be neglected and there may be a problem that the value is big in terms of the accuracy, and it is desirable that there is no hysteresis.